1. Field of the Invention
The present invention relates to a dental gypsum powder to be used for producing a dental model, forming a set material through kneading with water. More specifically, the invention relates to a dental gypsum powder having flowability on kneading, setting time, and strength of a set material that are suitable for dental use.
2. Description of the Conventional Art
In the field of dentistry, dental gypsum has been used for producing a model simulating an oral cavity, which is used in production of various dental prostheses to be installed in an oral cavity such as an inlay, a crown, a bridge, a partial denture and a complete denture.
The dental gypsum is generally available in the form of powder containing α-gypsum hemihydrate (hard gypsum) and/or β-gypsum hemihydrate (calcined gypsum), which are gypsum hemihydrate, as a major component. A dentist or a dental technician places prescribed amounts of dental gypsum powder and water for kneading in a small rubber bowl and kneads them with a dedicated spatula to prepare a gypsum slurry, and the gypsum slurry is then cast and set in a mold having an impression of an oral cavity, thereby producing a working model, a maxillary model and the like. A dental prosthesis is produced based on the working model, maxillary model and the like.
Accordingly, the dental gypsum is required to have accuracy capable of reproducing details of an oral cavity and is simultaneously required to have such a strength that prevents the reproduced details from being broken or damaged. The amount of water mixed is necessarily suppressed to a certain extent for ensuring the strength, and therefore a slurry of ordinary dental gypsum has a relatively high viscosity.
In view of the workability of the dental gypsum to be cast and set in a mold, a slurry of the dental gypsum is demanded to have flowability capable of flowing into the details of the mold. Furthermore, it is important to prevent air bubbles from contaminating into the slurry on kneading the dental gypsum powder and water since air bubbles having a diameter of approximately from 0.2 to 3 mm in the slurry may reduce the accuracy of the model. However, a slurry of ordinary dental gypsum is liable to be contaminated with air bubbles due to the high viscosity thereof as described above.
As a method for enhancing flowability of a composition kneaded with water, the use of a large powder-water ratio, which is the ratio of a kneading liquid to the powder, has been generally known. When the powder-water ratio is increased, however, there is a problem that the density of the set material is decreased after setting, and thus the mechanical strength thereof is decreased. For addressing the problem, it has been known in the field of building concrete that a water reducing agent or an AE water reducing agent is added to enhance flowability (see, for example, PTLs 1 to 3).
A water reducing agent is such a substance that facilitates kneading of powder and liquid to decrease the amount of water for kneading, thereby enhancing the strength of the composition after setting, and known examples thereof include lignin sulfonate, an oxycarboxylate salt, a naphthalenesulfonate salt, a melaminesulfonate salt, a polystyrenesulfonate salt and a polycarboxylate salt, which have a surface activation function. The use of these water reducing agent mixed decreases the viscosity of the slurry without changing the powder-water ratio, and thus the flowability may be improved to provide better kneading property and workability. The AE admixture (air entraining admixture) is a substance that has a function of dispersing minute independent air bubbles over the set material, in addition to the effects of the water reducing agent.
However, the use of the ordinary water reducing agent in dental gypsum powder considerably delays the setting thereof, and thus the ordinary water reducing agent has not been used in the field of dentistry, which requires a short setting time, for example, the set material is to be taken out from an impression within 5 minutes.
PTL 1
Japanese Patent No. 3,584,564
PTL 2
JP-A-2001-31457
PTL 3
JP-A-2006-282435